If you are thinking of purchasing conventional, wet type batteries, most Motorhomes will require two. One for starting and one for powering the Motorhomes domestic lighting, refrigeration, pumps and accessories.
For the best performance and life, the starting battery needs to be the type that allows high cranking amps with reserve for starting. If you are purchasing Wet Type Batteries, or Flooded Acid Type as they are sometimes called, they have more plates per size (thinner) than a Deep Cycle Battery to give higher cranking ability. They are not suitable for powering the domestic requirements, as they are not designed to be deep cycled and recharged.
They will fall apart internally in a short time if used for this service and are not a good investment in the long term. For the domestic requirements in a Wet Type Battery, a true Deep Cycle Battery is required for good life and reliability. These types of wet batteries have fewer (but thicker) plates and heavier duty separators for cyclic use, deep discharging and charging. They will give good service if charged and maintained properly. If charged properly, they will require water regularly.
There are some newer types of battery technologies available, which have definite advantages for motor caravaners over the wet types for both starting and deep cycling applications. For one, they do not require any maintenance at all, other than charging correctly (i.e. you never have to add water). They do not discharge themselves like wet batteries if left standing. For instance, they can be left for 6 months or more when fully charged and still be able to start your engine or power your Motorhome requirements. Conventional wet batteries will discharge themselves as much as 5% or more per week, whereas the newer technology types discharge themselves less than 1% per month! The wet Deep Cycle type will require an Equalisation Charge regularly to get good cyclic life. The newer technology types do not require any equalisation to achieve long life and good cyclic ability. These new technology batteries are generally referred to as VRLA (Valve Regulated Lead Acid), Sealed Type or Recombination Batteries. There are two main types, GEL and AGM.
GEL Batteries are exactly as they are called. Their electrolyte or acid is combined with silica and other chemicals to “immobilise” the acid in gel form. They have the advantage of not stratifying the normally liquid acid, and generally do not require any equalising. They are quite efficient compared to wet batteries but require a very careful charging regimen, and will give good long life if charged and discharged within their prescribed regimen. They do not have, for their size, as high a cranking capability for starting, and cannot be charged as quickly, or accept charge as well as wet type or AGM. GEL types tend to be more expensive than either wet or AGM batteries.
AGM stands for Absorbed Glass Mat and are sometimes referred to as Starved Electrolyte Batteries. They have specially constructed plates, which have been wrapped with a strong, micro-porous glass mat, and the electrolyte is held tightly to the plates. These plates are also compacted within the battery, which makes them very strong and rugged. There are a number of different varieties of this type of battery, but the better ones have the ability to operate in both a deep cycle mode with very high cranking ability for starting, and also have the ability to accept charging quickly and efficiently. In other words, they are a true dual purpose battery, and you do not have to buy two different types of batteries which require different charging regimens to properly keep them charged. Another plus is that they are a very safe battery as they are allowed to be carried on aircraft and they are not classified as Dangerous Goods. Both GEL and AGM do not give off gas when charged normally within their specifications, so are not dangerous like wet batteries when charging and discharging. They can be safely mounted anywhere inside, and do not have to be upright.
All being equal, AGM’s will give better service, longer life and cost less to operate in the long run. Additionally, if you are considering putting Solar charging on your Motorhome, the additional charging efficiency gained and the low self discharge of AGM’s ensures they will more than pay for themselves in the long run.